In the process of transportation, it is difficult to directly control the movement of the lifted-objects, which reflects the underactuated characteristics and brings many challenges. In recent years, the control for various underactuated systems, such as cranes and underwater vehicles, has received considerable interest and achieved satisfactory results by utilizing nonlinear control methods [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}. With the application of advanced sensors, feedback control for the intractable drone has recently attracted extensive attention[9]}, [10]}, [11]}, [12]}. Some classic control strategies are used to realize various tasks such as aggressive maneuvers, dynamic trajectory generation, and so on. As for the drone transportation system, the load cannot be directly controlled, and the cables exacerbate the system nonlinearities and introduce complex dynamic couplings, these factors make the control problem for the drone transportion system full of all kinds of challenges. So far, researchers have done a lot of work to suppress and eliminate the load sway efficiently, with effective methods including geometric control[13]}, dynamic programming[14]}, differential flatness[15]}, time-optimal motion planning[16]}, hierarchical control[17]}, and so on. Regardless of the control strategy, the transportation capacity of a single drone is always limited, it is thus urgently needed to implement transportation tasks by multiple drones with high-performance cooperative control strategy.
